I had about the same background as your friend (C + shell/awk + many others during my CS studies). When I first started using it I thought that Perl felt very, _very_ natural. It is really easy to write Perl that looks a bit like C, and thinks like using foreach instead of the clumsy C loops and pepperring the code with regexps are a welcome simplification.
The things that I found the most difficult to understand: map, some tricky (and non-standard) regexps features like lookahead/lookbehind, the difference between scalar and list context. It also took me a while to get used to $_.
Overall your friend should have no problem getting into Perl itself. Then it really depends on the quality of the existing code he has to work with... Getting the authors, if they are still with the customer, to explain the design of their code and their coding style would certainly be of great help.
